Conventionally, for example, in order to achieve both safety in the event of collision, and a vehicle weight reduction for fuel cost improvement, high-tensile steel is used for structural members and reinforcement members for a vehicle. However, when the high-tensile steel plate is press-molded with dies during manufacture, a processed product may, for example, spring back (i.e., return to its original shape after deformed), and suffer from insufficient shape retention after press-molding, making it difficult to manufacture a good quality product. To overcome the foregoing, the shapes of dies are modified in advance taking account of the degree to which a processed product may spring back, and the like, or the number of steps for modifying the shape is increased, which is not economical.
Therefore, in order to increase the strength of a pressed component, the following has been developed: a method (see, e.g., Patent Document 1) in which a heated steel plate is press-molded by dies while a hot condition is maintained (e.g., hot pressing) and is quenched while in the dies to achieve a high-tensile steel; and a technology for high-frequency quenching. However, high-frequency quenching requires considerable know-how in the quenching and cooling method, and has not been generally used. Hot pressing, in which the foregoing problems occur relatively rarely, is widespread and has become a general technology.
However, the conventional hot pressing includes: heating a material to a temperature (about 950° C.) lower than its melting point by equipment such as a heating furnace before press processing; and molding the material between the upper and lower dies by press processing and, at the same time, carrying out quenching through sudden cooling. Thereby, a product with the considerable strength of about 1500 MPa is obtained. However, although the product thus obtained is rendered very strong, it is a form of iron and, therefore, is not different in Young's modulus from low-tensile-strength iron, which is generally called steel. Once the material is determined, its static rigidity is determined according to the plate thickness. Accordingly, rendering the material very strong does not always allow the material to be thinned readily. Therefore, conventionally, the use of hot pressing is limited to products that are originally sufficiently rigid or to components that can be made more rigid through a design technique such as increasing cross-sectional area or modifying cross-sectional shape.